This invention relates to single pressure circuit interrupters and more specifically relates to a circuit interrupter using sulfur hexafluoride in the high density state and preferably in the liquid state wherein the arc energy of the arc produced between the separating contacts is used to cause the fluid sulfur hexafluoride to sweep rapidly through the separating contacts in order to deionize the arc drawn therebetween and further to provide at least part of the energy required to separate the cooperating contacts.
It is well known that the current interrupting ability of a sulfur hexafluoride interrupter can be increased by increasing the gas pressure of the sulfur hexafluoride. In fact, it is known that interrupters using sulfur hexafluoride as the arc interrupting medium can have the gas pressure increased well above its critical pressure of 537 PSI and in fact can be as high as 5800 PSI. A disclosure of a circuit interrupter operating at this relatively high pressure is contained in U.S. Pat. No. 3,842,227 entitled CIRCUIT BREAKER HAVING DIELECTRIC LIQUID UNDER PRESSURE.
It is also known to use sulfur hexafluoride at supercritical pressure in puffer type circuit breakers. Thus a single piston type interrupter of the type shown in U.S. Pat. No. 4,009,358 entitled ELECTRIC CIRCUIT-BREAKER FOR ALTERNATING CURRENTS and a dual piston type interrupter of the type shown in Application Ser. No. 843,573, filed Oct. 19, 1977 in the name of R. D. Garzon, entitled LIQUID SF.sub.6 PUFFER TYPE CIRCUIT INTERRUPTER can be used. In these devices interruption is obtained by causing one or more pistons to be moved while separating a pair of electric contacts in order to initiate an arc and at the same time forcing the sulfur hexafluoride fluid to flow through the arcing region to enhance arc quenching.
Single piston designs utilizing sulfur hexafluoride in the past have been subject to several disadvantages, particularly when the sulfur hexafluoride is at subcritical temperature and thus in its liquid state. Thus, in such devices, very high energy is required for the driving mechanism; an extremely high pressure surge is produced during high current arcing; and there is a relatively low dielectric withstand after low current interruption at low temperature. High energy is required for the driving mechanism since the dielectric fluid is a high density and high viscosity fluid at supercritical pressures. The high pressure surge during high current arcing is attributed to the low compressibility of liquid sulfur hexafluoride. Moreover, at maximum arcing duty a pressure surge due to the arcing could reach several thousand pounds per square inch, thus creating serious mechanical problems within the interrupter. At the other end of the spectrum, of interruption of extremely low current and low temperature, there is a substantial decrease in the pressure of the sulfur hexafluoride and therefore a decrease in dielectric withstand due to the expansion of the sulfur hexafluoride volume during the circuit interrupter operation.
When using a dual piston arrangement with the pistons disposed on opposite sides of the interrupting contacts, the sulfur hexafluoride volume within the interrupter is virtually constant since identical piston diameters on both sides of the contacts are usually used. Therefore, there will be no decrease in dielectric withstand capability after interruption. However, the problem of a high pressure surge due to arcing will be more severe than in the case of the single piston design. Moreover, the driving energy requirement for the dual piston design will still be very high.